1. Field of the Invention
The invention relates generally to the field of seismoelectric and electroseismic surveying of the Earth's subsurface. More specifically, the invention relates to methods for acquiring electroseismic and seismoelectric surveys such that subsurface features may be more readily identified.
2. Background Art
Electromagnetic geophysical surveying known in the art includes “controlled source” electromagnetic surveying. Controlled source electromagnetic surveying includes imparting an electric field or a magnetic field into the Earth formations, those formations being below the sea floor in marine surveys, and measuring properties of electric fields and/or magnetic fields induced in response. The electric and/or magnetic field properties are measured by measuring voltages induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth's surface, or on or above the sea floor. The electric and/or magnetic field properties are used to make inferences about the spatial distribution of electrical conductivity of the Earth's subsurface.
Controlled source electromagnetic surveying known in the art includes imparting alternating electric current into formations below the sea floor. The alternating current has one or more selected frequencies. Such surveying is known as frequency domain controlled source electromagnetic (f-CSEM) surveying. f-CSEM surveying techniques are described, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R., 1990, An active source electromagnetic sounding system for marine use, Marine Geophysical Research, 12, 29-68. Other publications which describe the physics of and the interpretation of electromagnetic subsurface surveying include: Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes, D. C., Bone, M. N., Trigg, D. F., and DeLaurier, J. M., 1985, First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding: Geophysics 50, No. 1, 153-160; Edwards, R. N., 1997, On the resource evaluation of marine gas hydrate deposits using the sea-floor transient electric dipole-dipole method: Geophysics, 62, No. 1, 63-74; Chave, A. D., Constable, S. C. and Edwards, R. N., 1991, Electrical exploration methods for the seafloor: Investigation in geophysics No 3, Electromagnetic methods in applied geophysics, vol. 2, application, part B, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987, On the theory of sea-floor conductivity mapping using transient electromagnetic systems: Geophysics, 52, No. 2, 204-217.
Following are described several patent publications which describe various aspects of electromagnetic subsurface Earth surveying. U.S. Pat. No. 5,770,945 issued to Constable describes a magnetotelluric (MT) system for sea floor petroleum exploration. The disclosed system includes a first waterproof pressure case containing a processor, AC-coupled magnetic field post-amplifiers and electric field amplifiers, a second waterproof pressure case containing an acoustic navigation/release system, four silver-silver chloride electrodes mounted on booms and at least two magnetic induction coil sensors. These elements are mounted together on a plastic and aluminum frame along with flotation devices and an anchor for deployment to the sea floor. The acoustic navigation/release system serves to locate the measurement system by responding to acoustic “pings” generated by a ship-board unit, and receives a release command which initiates detachment from the anchor so that the buoyant package floats to the surface for recovery. The electrodes used to detect the electric field are configured as grounded dipole antennas. Booms by which the electrodes are mounted onto a frame are positioned in a configuration to create two orthogonal dipoles. The two orthogonal dipoles are used to measure the complete vector electric field. The magnetic field sensors are multi-turn, Mu-metal core wire coils which detect magnetic fields within the frequency range typically used for land-based MT surveys. The magnetic field coils are encased in waterproof pressure cases and are connected to the logger package by high pressure waterproof cables. The logger unit includes amplifiers for amplifying the signals received from the various sensors, which signals are then provided to the processor which controls timing, logging, storing and power switching operations. Temporary and mass storage is provided within and/or peripherally to the processor. There is no active source in such MT methods, which rely upon naturally occurring EM fields.
U.S. Pat. No. 6,603,313 B1 issued to Srnka discloses a method for surface estimation of reservoir properties, in which average earth resistivities above, below, and horizontally adjacent to specifically located subsurface geologic formations are first determined or estimated using geological and geophysical data in the vicinity of the subsurface geologic formation. Then dimensions and probing frequency for an electromagnetic source are determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation, using the location and the average earth resistivities. Next, the electromagnetic source is activated at or near the sea floor, approximately centered above the subsurface geologic formation and a plurality of components of electromagnetic response is measured with a receiver array. Geometrical and electrical parameter constraints are determined, using the geological and geophysical data. Finally, the electromagnetic response is processed using the geometrical and electrical parameter constraints to produce inverted vertical and horizontal resistivity depth images. Optionally, the inverted resistivity depth images may be combined with the geological and geophysical data to estimate the reservoir fluid and shaliness (fractional volume in the formation of clay-bearing rocks called “shale”) properties.
U.S. Pat. No. 6,628,110 B1 issued to Eidesmo et al. discloses a method for determining the nature of a subterranean reservoir whose approximate geometry and location are known. The disclosed method includes: applying a time varying electromagnetic field to the strata containing the reservoir; detecting the electromagnetic wave field response; and analyzing the effects on the characteristics of the detected field that have been caused by the reservoir, thereby determining the content of the reservoir, based on the analysis.
U.S. Pat. No. 6,541,975 B2 issued to Strack discloses a system for generating an image of an Earth formation surrounding a borehole penetrating the formation. Resistivity of the formation is measured using a DC measurement, and conductivity and resistivity of the formations are measured with a time domain signal or AC measurement. Acoustic velocity of the formation is also measured. The DC resistivity measurement, the conductivity measurement made with a time domain electromagnetic signal, the resistivity measurement made with a time domain electromagnetic signal and the acoustic velocity measurements are combined to generate the image of the Earth formation.
International Patent Application Publication No. WO 0157555 A1 discloses a system for detecting a subterranean reservoir or determining the nature of a subterranean reservoir whose position and geometry is known from previous seismic surveys. An electromagnetic field is applied by a transmitter on the seabed and is detected by antennae also on the seabed. A refracted wave component is sought in the wave field response, to determine the nature of any reservoir present.
International Patent Application Publication No. WO 03048812 A1 discloses an electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir. The method includes obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination so as to allow the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the effects of signal attenuation (which are highly dependent on local properties of the rock formation, overlying water, and air at the survey area). This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of subsurface formations.
U.S. Pat. No. 6,842,006 B1 issued to Conti et al. discloses a sea-floor electromagnetic measurement device for obtaining underwater magnetotelluric (MT) measurements of earth formations. The device includes a central structure with arms pivotally attached thereto. The pivoting arms enable easy deployment and storage of the device. Electrodes and magnetometers are attached to each arm for measuring electric and magnetic fields respectively, the magnetometers being distant from the central structure such that magnetic fields present therein are not sensed. A method for undertaking sea floor measurements includes measuring electric fields at a distance from the structure and measuring magnetic fields at the same location.
U.S. Patent Application Publication No. 2004/232917 relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the Earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The system and method enable subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.
U.S. Pat. No. 5,467,018 issued to Rueter et al. discloses a bedrock exploration system. The system includes transients generated as sudden changes in a transmission stream, which are transmitted into the Earth's subsurface by a transmitter. The induced electric currents thus produced are measured by several receiver units. The measured values from the receiver units are passed to a central unit. The measured values obtained from the receiver units are digitized and stored at the measurement points, and the central unit is linked with the measurement points by a telemetry link. By means of the telemetry link, data from the data stores in the receiver units can be successively passed on to the central unit.
U.S. Pat. No. 5,563,913 issued to Tasci et al. discloses a method and apparatus used in providing resistivity measurement data of a sedimentary subsurface. The data are used for developing and mapping an enhanced anomalous resistivity pattern. The enhanced subsurface resistivity pattern is associated with and an aid for finding oil and/or gas traps at various depths down to a basement of the sedimentary subsurface. The apparatus is disposed on a ground surface and includes an electric generator connected to a transmitter with a length of wire with grounded electrodes. When large amplitude, long period, square waves of current are sent from a transmission site through the transmitter and wire, secondary eddy currents are induced in the subsurface. The eddy currents induce magnetic field changes in the subsurface which can be measured at the surface of the earth with a magnetometer or induction coil. The magnetic field changes are received and recorded as time varying voltages at each sounding site. Information on resistivity variations of the subsurface formations is deduced from the amplitude and shape of the measured magnetic field signals plotted as a function of time after applying appropriate mathematical equations. The sounding sites are arranged in a plot-like manner to ensure that areal contour maps and cross sections of the resistivity variations of the subsurface formations can be prepared.
A limitation to f-CSEM techniques known in the art is that they are typically limited to relatively great water depth, on the order of 800-1,000 meters, or a ratio of ocean water depth to subsurface reservoir depth (reservoir depth measured from the sea floor) of greater than about 1.5 to 2.0.
A typical f-CSEM marine survey can be described as follows. A recording vessel includes cables which connect to electrodes disposed near the sea floor. An electric power source on the vessel charges the electrodes such that a selected magnitude of alternating current, of selected frequency or frequencies, flows through the sea floor and into the Earth formations below the sea floor. At a selected distance (“offset”) from the source electrodes, receiver electrodes are disposed on the sea floor and are coupled to a voltage measuring circuit, which may be disposed on the vessel or a different vessel. The voltages imparted into the receiver electrodes are then analyzed to infer the structure and electrical properties of the Earth formations in the subsurface.
Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying (t-CSEM). In t-CSEM, electric current is imparted into the Earth at the Earth's surface (or sea floor), in a manner similar to f-CSEM. The electric current may be direct current (DC). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching strategies are possible; as will be explained in more detail below. Structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields. t-CSEM techniques are described, for example, in Strack, K.-M., 1992, Exploration with deep transient electromagnetics, Elsevier, 373 pp. (reprinted 1999).
Other techniques used to make inferences about the structure and composition of the formations in the Earth's subsurface include seismoelectric surveying and electroseismic surveying. Electroseismic surveying is described in U.S. Pat. No. 5,486,764 issued to Thompson et al., which method and apparatus for estimating the Earth's resistance (conductivity) as a function of depth. Resistance is determined by the frequency-dependent attenuation of reflected electromagnetic (EM) signals which are produced by application of seismic signals to the Earth. A seismic wave is applied by conventional means into the Earth, and EM waves propagate back to the Earth's surface from different reflectors at different depth levels in the subsurface. The propagation back through the Earth attenuates the high frequencies preferentially. EM waves generated at lower depths are more attenuated relative to those waves generated at shallower depths. The method and apparatus described in the Thompson '764 patent determines the difference in spectral content between the reflected EM signals from different horizons based on their relative attenuation, and uses these differences as a direct measure of the conductivity between the reflective horizons. Inverse electroseismic prospecting techniques can be used to impart EM waves into the Earth and use reflected seismic waves to determine electrical resistance as a function of depth. The '764 patent also describes the inverse process, called seismoelectric prospecting, in which seismic waves are imparted into the Earth and induced voltages are detected in response thereto.
U.S. Pat. No. 5,841,280 issued to Yu et al. describes a seismoelectric well logging instrument and method for using such instrument. The method disclosed is for estimating porosity of an earth formation from measurements of acoustic energy traversing the earth formation and from measurements of seismoelectric voltages generated in the formation in response to the acoustic energy. The method includes the steps of measuring the acoustic energy traversing the earth formation and measuring the seismoelectric voltages generated in response to the acoustic energy traversing the formation. A seismoelectric signal is synthesized from the measurements of the acoustic energy using an initial value of the porosity. A difference is determined between the synthesized seismoelectric voltages and the measured seismoelectric voltages. The initial value of porosity is adjusted, and the steps of synthesizing the seismoelectric voltages from the acoustic signal, determining the difference, and adjusting the value of porosity are repeated until the difference drops below a predetermined threshold or the difference reaches a minimum value. The adjusted value of porosity which results in the difference being at the minimum is taken as the formation porosity. In a particular embodiment, the electrical conductivity of fluid in pore spaces of the earth formations can be calculated. The particular embodiment includes the steps of measuring the acoustic energy traversing the earth formation and measuring the seismoelectric voltages generated in response to the acoustic energy traversing the formation. A seismoelectric signal is synthesized from the measurements of the acoustic energy using an initial value of fluid electrical conductivity. A difference is determined between the synthesized seismoelectric voltages and the measured seismoelectric voltages. The initial value of fluid electrical conductivity is adjusted, and the steps of synthesizing the seismoelectric voltages from the acoustic signal, determining the difference, and adjusting the value of conductivity are repeated until the difference drops below a predetermined threshold or the difference reaches a minimum value. The adjusted value of fluid electrical conductivity which results in the difference being at the minimum is taken as the fluid electrical conductivity.
The origin of seismoelectric signals is shown in FIG. 1A and FIG. 1B. FIG. 1A shows how Stoneley waves traveling through a borehole drilled through a permeable Earth formation can displace electrically conductive fluid in the pore spaces of the formation. Displacement of the fluid in the pore spaces causes an electric field to be generated. FIG. 1B shows Stoneley waves moving along a borehole that penetrates a fractured Earth formation. Fluid in the fracture is displaced by the action of the Stoneley waves and generates an electromagnetic field in response. FIG. 2A shows a seismic (acoustic pressure or compressional wave) reflection response from the Earth to imparted seismic energy. FIG. 2B shows seismoelectric response of the same region of the Earth's subsurface, showing a degree of correspondence between seismic response and seismoelectric response of the Earth.
Adapting the foregoing electroseismic and seismoelectric techniques to surveying from the Earth's surface or from the surface of a body of water is known in the art. Each of the techniques described above, including controlled source electromagnetic surveying, electroseismic surveying and seismoelectric surveying, and although not described above, but equally applicable to this description, seismic surveying known in the art, are each responsive to different characteristics of the formations in the Earth's subsurface. Combining the results of all of the foregoing survey techniques can provide improved analysis of the structure and composition of the formations in the Earth's subsurface. However, conducting four or more different types of surveys over the same area of the Earth's subsurface can be time consuming and expensive. Each of the foregoing techniques also investigates different volume in the Earth's subsurface with respect to the placement of the respective energy sources and detecting devices used therewith. Such differing volumes of investigation have made combining acquisition and combining results of the techniques to be impractical. What is needed is a method and system to combine various surface electric and seismic survey techniques to improve the quality of analysis of the Earth's subsurface. At the same time, integration of such interpretations with borehole or fluid injection/production measurements is required to provide results that are grounded in actual subsurface measurements, and/or fluid production information.